Subduction Processes Along the Peruvian Margin: Results From Wide-Angle Seismic Studies

Within the scope of the GEOPECO (Geophysical Experiments at the Peruvian Continental Margin) project, active seismic investigations along the Pacific margin of Peru were carried out using ocean bottom hydrophones (OBH) and seismometers (OBS) recording marine airgun shots. The structure and the P-wave velocity of the oblique subducting Nazca and overriding South-American plates from 8° S to 15° S were determined by forward modeling and tomographic inversion of the wide-angle seismic data combined with the analysis of reflection seismic data. The region south of 12° S has been influenced by the southward migration of the aseismic Nazca Ridge the past 11 Myrs. Mendana Fracture Zone (MFZ) on the oceanic Nazca plate marks a transition zone of a different crustal age of approximately 28 Ma in the north to 38 Ma in the south at the Peruvian trench. The oceanic crust of the Nazca plate, with a thin pelagic sediment cover, has an average thickness of 6.4 km. An intracrustal layer can be observed across the entire Peruvian margin. The P-wave velocities of the upper crustal layer displays values from 4.7-6.1 km/s and the refractions of the lower crustal layer show velocities from 6.4-7.3 km/s. The P-wave velocity of the mantle is overall 7.9-8.1 km/s. The steep lower slope of the continental margin is associated with the frontal prism of 12-15 km width, displaying low seismic P-wave velocities of 1.9-2.0 km/s. These frontal sediments accrete against a backstop structure which is defined by higher seismic velocities of 3.5-6.0 km/s. Basal erosion is inferred by the low seismic velocities of the frontal prism, the large variations in the surface slope and the bending of the subducting slab. A subduction channel cannot be resolved by forward modeling of the refraction data but amplitude modeling indicates a low seismic velocity layer on top of the subducting slab.